Ophthalmic progressive lenses allow the wearer to benefit from an optical power compensation that is adapted to various vision distances without changing spectacles. They may also correct other visual defects, such as astigmatism for example.
A progressive ophthalmic lens has a variable power over the surface of the lens.
For example provision is made for a first vision zone for far vision having a first average power value, a second vision zone for near vision having a second average power value and, between these two zones, a third vision zone for intermediate vision, the curvature of which varies gradually and which is called the progression corridor.
The difference between the first and second average power values is equal to the power addition of the lens.
The fitting cross is a reference point for positioning the lens in front of the eye of a wearer, the position of which is predefined by the manufacturer of the lens.
The first zone for far vision is centered on a reference point for far vision, the position of which is predetermined for a given lens by the manufacturer.
The second zone for near vision is centered on a reference point for near vision, the position of which is predetermined for a given lens by the manufacturer.
The first zone for far vision and the second zone for near vision are separated by a distance called progression length.
The progression length may be defined as the vertical distance between the fitting cross and the position of the near-vision reference point defined by the manufacturer of the eyeglass.
The vertical and horizontal directions of the lens are defined depending on the position of the lens under conditions of use by the wearer, in the chosen frame.
The progression length of the lens must be adjusted depending on the fitting height of the ophthalmic lens.
The fitting height of the ophthalmic lens corresponds to the height, relative to the lower edge of the rim of the frame, of the projection of the pupil of the wearer having a predetermined primary gaze direction onto a mean plane of this rim of the chosen frame, corresponding to a mean plane of the ophthalmic lens once fitted into said frame.
This predetermined primary gaze direction corresponds to the gaze direction of the wearer under far-vision conditions.
The progression length of the lens is adjusted so that the second zone for near vision of the lens is included in the lens once edged and positioned in the chosen frame.
Furthermore, the second zone used for near vision may be positioned depending on visual habits of the wearer.
Customarily, the choice of the progression length is made by the optician on the basis of subjective criteria such as the posture of the wearer or feedback given by the latter on his past piece of equipment.
A method for determining the progression distance of a lens is also known from document U.S. Pat. No. 8,297,752, in which method a single far-vision point of the wearer and a single near-vision point of the wearer are determined on the ophthalmic lens and the corresponding progression length is deduced therefrom. An ophthalmic lens suitable for the wearer may thus be selected.
However, it is not certain, by applying this method, that a sufficient portion for comfortable use of the second zone for near vision used by the wearer will be included in the ophthalmic lens once the latter has been edged and fitted in the frame chosen by the wearer. Specifically, generally, the precise determination of the progression length involves a precise determination of the position of the first and second vision zones. The precise positioning of the frame on the face of the wearer, resulting for example from the geometry of the frame and the fit of this frame on the head of the wearer, directly influences the position of the first and second vision zones for a given wearer. These parameters are not taken into account in the prior art.